1. Field of the Invention
The present invention relates in general to friction-reducing ships and methods for reducing skin-friction, and relates in particular to a technology for reducing skin-friction by blowing micro-bubbles from the hull into water.
2. Description of the Related Art
Technologies for reducing skin-friction in ships are disclosed in a number of Japanese Patent Applications, First Publications, S50-83992, S53-136289, S60-139586 and S61-71290 and in Practical Utility Model Applications, S61-39691, S61-128185, for example. These technologies are based on reducing skin-friction of a ship by blowing gas from the hull surface into water to introduce numerous bubbles in an interface between hull and water to reduce the skin-friction between the ship and water.
In such technologies, it is known that, to increase the friction reduction effects created by the bubbles, the volume of gas blowing into the water should be increased to raise the average void fraction .alpha..sub.m (concentration of micro-bubbles) in the turbulent boundary layer formed on the hull surfaces, but it is found that higher gas flow rates cause micro-bubbles to be pushed out of the turbulent boundary layer, resulting in no increase in the average void fraction .alpha..sub.m within the boundary layer.
A practical method that can be applied to a friction-reducing ship is to produce bubbles by blowing compressed air generated by on-board compressors to produce a desired void fraction in a boundary layer at the bottom section of a ship.
However, because of a high static pressure at the bottom section of a ship, such technologies are highly energy-consuming, and the energy required to generate micro-bubbles exceeds the energy saving resulting from reducing the skin-friction, and adaptation of bottom-bubbles technology to practical situations becomes problematic.
Through a series of investigations into skin-friction effects in ships, the present inventors have discovered that friction-reducing ships can be made practical, if jetting nozzles are located in low static pressure regions and the bubbly flow fields created at the bottom and hull surfaces of the ship can be made to flow along the hull surfaces. Based on such a premise, the present inventors developed a computational model in which the bubbles are generated in low static pressure regions, and given the shape of a hull structure, the model is able to compute void fractions in any locations about the hull structure by considering the turbulent diffusion of micro-bubbles along the flow lines near the hull surfaces.
According to the computational equations, developed using turbulence coefficients in assumed isotropic diffusion fields, the effects of turbulent diffusion are considered by varying the flow speeds in X-, Y- and Z-directions (levitation direction) so as to create turbulence in the traces of micro-bubbles flowing about the hull surfaces. In other words, random activities of the bubbles are simulated directly by using Monte Carlo methods. When the activities of the micro-bubbles are so determined, a void fraction at a given point in time can be obtained by dividing the volume of the micro-bubbles existing in a given volume of an inspection volume (cell volume) by the volume of the cell. Based on the distribution patterns of the void fraction thus obtained, optimum configuration of the gas jetting devices, located at the bow of a ship where maximum friction reduction effects are expected, can be determined in relation to the flow lines originating in those locations and spreading along the hull surfaces towards the stern of the ship.
The bubbles ejected from the bow of the ship into the boundary layer are carried along with the flow lines in such a way that those bubbles in the leading regions of the bottom section will flow along the bottom surface, but those bubbles in the latter regions of the ship will tend to flow along the lateral surfaces of the ship. The ultimate result is that the bubbles are able to blanket the entire submerged surfaces of the moving ship to provide effective friction reduction. The micro-bubbles blanketing the hull surfaces can contribute more effectively to friction reduction if they can cling to the hull surfaces.
However, although those bubbles which are carried to the bottom section dwell near the bottom surface, for a time, and cling to the bottom surface, those lateral bubbles which are flowing along the lateral hull of the ship cannot effectively redirect the lift force inherent in the bubbles themselves to the clinging direction. This means that there is little force being exerted on those lateral bubbles to keep them near the hull surfaces. In other words, for a general shape of a ship represented typically by a tanker which has the side hull plates attached at roughly right angles to the bottom plate, the lateral bubbles do not generate a force to keep the bubbles close to the surface. Therefore, it is considered by the present inventors that the friction reduction effects can be further improved if the bubbles can be subjected to a lateral force to keep the bubbles clinging to the hull surfaces.
Furthermore, for the bottom bubbles, the larger the bubble diameter the greater the floating force to keep themselves clinging to the bottom surface, and the average void fraction in the boundary layer is higher and the greater friction reduction effects are produced. However, for the lateral bubbles, the force to keep the bubbles close to the hull surface is only the lift force (Saffman's lift) derived from a difference between the shear flow speed and the bubble flow speed. The magnitude of this lift force is relatively small and the bubbles are quickly carried away from the hull surface. This behavior of the lateral bubbles means that they cannot contribute effectively to reducing the skin-friction, because, a boundary layer is generally formed thin near the bow and becomes thicken towards the stern of a ship, therefore, if the gas flow rate through the gas jetting device is adjusted to maximize bubble retention in a thick boundary layer, the bubbles are blown out of the boundary layer from a thinner boundary layer. Therefore, it is critical to control bubble generation conditions so as to retain the bubbles within the boundary layer under all operating conditions of the ship to maximize the friction reduction effects of the micro-bubbles.